Edit: Additional links added!

Here's a small compilation of super useful but somewhat hidden wiki pages. If you have any useful wiki pages that aren't linked on the home page, please do share and I'll add it to the list.

All category pages

(Thanks u/salienzero)

A page listing all category pages, such as some of the category pages listed below in this post:

https://exvius.gamepedia.com/Category:Abilities

Unit comparison chart

(Thanks u/delta_angelfire)

A comprehensive page that covers buffs, debuffs, ailments, mitigations and more in sortable tables:

https://exvius.gamepedia.com/Unit_Comparison_Charts

Elements

Sortable tables for elemental resistance buffs and debuffs (aka imperils) for each element:

https://exvius.gamepedia.com/Category:Elements

Individual element pages

(Thanks u/ksuwdboots)

Fire: https://exvius.gamepedia.com/Category:Fire

Ice: https://exvius.gamepedia.com/Category:Ice

etc.

Stats

Sortable tables for stat buffs and debuffs (aka breaks) for each stat:

https://exvius.gamepedia.com/Category:Stats

Killers

Sortable tables for killers by race & damage type. Includes innate killers, killers granted by equipment, and killers granted by buffs:

(u/RomanticTitan warns that this page may not be current)

https://exvius.gamepedia.com/Category:Killer

Mitigation

Sortable tables for cover and mitigation buffs by type, ST/AoE, and duration:

https://exvius.gamepedia.com/Category:Mitigation

Evasion

(Thanks u/SirBarth)

Sortable tables for innate and equipment physical and magical evasion:

https://exvius.gamepedia.com/Category:Evasion

Ailments

(Thanks u/CottonC_3939)

Sortable table for inflicting or preventing ailments:

https://exvius.gamepedia.com/Category:Ailments

Update schedule

(Thanks u/fourrier01)

A page that helps you answer "What's coming up next?":

https://exvius.gamepedia.com/Update_Schedule

JP unit list

(Thanks u/fourrier01)

A list of current JP units:

https://exvius.gamepedia.com/Unit_List/JP

Global changes to Units

(Thanks u/secace)

A list of GL changes to units:

https://exvius.gamepedia.com/Global_changes_to_units

u/juliusbenedikt confirms that the page is still maintained.

List of TDH/TDW sources

(Thanks u/CottonC_3939)

A page listing TDH/TDW unit passives & gear:

https://exvius.gamepedia.com/Category:Equipment_Stats

Mechanics

(Thanks u/Neopatrimonialism)

A comprehensive page regarding all FFBE gear & combat mechanics:

https://exvius.gamepedia.com/Mechanics